Long-term evolution (LTE) system supports two duplex modes including frequency division duplex (FDD) and time division duplex (TDD).
FIG. 1 shows a frame structure of a TDD system according to the related art.
Referring to FIG. 1, each radio frame is of 10 ms length and is divided into two 5 ms half-frames. Each half-frame includes eight 0.5 ms slots and three special fields, i.e., downlink pilot slot (DwPTS), guard period (GP) and uplink pilot slot (UpPTS). The total length of the three special fields is 1 ms. The TDD system supports 7 kinds of uplink-downlink configurations, as shown in Table 1. Herein, D denotes a downlink subframe, U denotes an uplink subframe, and S denotes a special subframe including the above three special fields.
TABLE 1Uplink-downlink configurations of LTE TDDConfig-SwitchingurationpointSubframe indexindexperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
In the LTE-advanced (LTE-A) system, a wider working bandwidth is obtained through combining multiple component carriers (CCs) via a carrier aggregation (CA) technique, and therefore data transmission rate may be further increased. Each CC corresponds to one cell. According to current UE standards (e.g., Rel-12), a UE may work on at most 5 CCs at the same time, wherein one of the 5 CCs is a primary cell (Pcell), and other CCs are secondary cells (Scells).
In downlink communication of the LTE-A system, a hybrid automatic repeat request (HARQ) technique is utilized to ensure reliability of downlink data receipt. The user equipment (UE) receives a downlink grant (DL-GRANT), wherein the DL-GRANT is carried by a physical downlink control channel (PDCCH) or an enhanced PDCCH (EPDCCH). For each transmission block (TB) received via physical downlink shared channel (PDSCH), or received PDCCH indicating release of semi-persistent scheduling (hereinafter the above two are referred to as downlink HARQ transmission), the UE needs to feed back an acknowledgement (ACK) (correct receipt) bit or negative ACK (NACK) (incorrect receipt or lost) bit to the base station via corresponding uplink subframe, hereinafter referred to as a HARQ-ACK bit. If the eNB receives the NACK bit, the eNB re-transmits the TB corresponding to the NACK or the PDCCH indicating the release of the SPS. According to different HARQ-ACK mechanisms, the LTE-A standard defines corresponding methods for determining the number of HARQ-ACK bits to be fed back and values of the HARQ-ACK bits.
In an FDD system, if the UE feeds back a HARQ-ACK bit in an uplink subframe n via a physical uplink shared channel (PUSCH), the number of HARQ-ACK bits to be fed back is determined according to the number of carriers configured for the UE and a transmission mode (e.g., transmission mode of one TB or transmission mode of two TBs) of each carrier. For each carrier configured for the UE, if the transmission mode is one TB, the carrier corresponds to one HARQ-ACK bit. If the transmission mode is two TBs, the carrier corresponds to two HARQ-ACK bits. The bits are arranged according to an ascending order of the indexes of the carriers, to form a HARQ-ACK bit sequence that the UE finally feeds back in an uplink subframe n. The HARQ-ACK bit sequence refers to that before channel coding, and the same applies hereinafter.
In the TDD system, the number of HARQ-ACK bits to be fed back by the UE in an uplink (UL) subframe n is determined by a HARQ-ACK time-frequency bundling window, a UL downlink assignment index (UL DAI) carried in a UL grant (UG) of subframe n, a number of carriers configured for the UE, and the transmission mode configured for each carrier, in which the HARQ-ACK time-frequency bundling window is determined by a TDD uplink-downlink configuration corresponding to a HARQ-ACK timing relationship followed by the HARQ-ACK feed back of the UE, denoting all downlink subframes on one carrier whose HARQ-ACK is to be fed back in a subframe n. The indexes of the downlink subframes are denoted by n-ki, ki∈K, wherein the dimension M of the set K is referred to as the size of the time-frequency bundling window. The set K determined by the present LTE standard with respect to the HARQ timing relationships corresponding to different TDD uplink-downlink configurations is as shown in Table 2. For facilitating the description, the subframe set K corresponding to the time-frequency bundling window determined by the HARQ timing relationship of FDD is defined as {4}, M=1 at this time.
In the following, UL DAI denotes a maximum number of downlink subframes that actually have downlink HARQ transmission in the time-frequency bundling window configured for each carrier of the UE. For each carrier configured for the UE, the number of downlink subframes that need to teed back HARQ-ACK in a subframe n is Bc=min (Mc, UL DAI), wherein min denotes an operation of obtaining a minimum value, and Mc denotes the size of the time-frequency bundling window corresponding to the carrier c. If the transmission mode of the current carrier is one TB, the number of HARQ-ACK bits corresponding to this carrier is Oc=Bc, and each subframe corresponds to one HARQ-ACK bit. If the transmission mode of the current carrier is two TBs, the number of HARQ-ACK bits corresponding to the carrier is Oc=2×Bc, and each downlink subframe corresponds to two HARQ-ACK bits.
TABLE 2Set K: {k0, k1, . . . kM−1} determinedby different HARQ timing relationshipsTDD uplink-downlinkconfig-Subframe indexuration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7,————8, 7,——4, 64, 63——7, 6, 116, 55, 4—————4——12, 8,6, 5,——————7, 114, 75——13, 12,———————9, 8, 7,5, 4, 11,66——775——77—
In the TDD system, the HARQ-ACK bit sequence that needs to be fed back by the UE is determined by a sum OUE of HARQ-ACK bits corresponding to all carriers. If OUE is not larger than 20, the HARQ-ACK bit of each carrier is arranged according to an ascending order of the carrier indexes to form the HARQ-ACK bit sequence to be fed back by the UE. Otherwise, if OUE is larger than 20, for all carriers whose transmission mode is two TBs, an “OR” calculation (i.e., spatial bundling) is performed on the two HARQ-ACK bits corresponding to two TBs of each subframe, to obtain one HARQ-ACK bit. For the carriers whose transmission mode is one TB, the HARQ-ACK bit corresponding to each subframe is unchanged. After the above processing, the HARQ-ACK bit of each carrier of the UE is arranged according to the ascending order of the carrier indexes to generate the HARQ-ACK bit sequence to be fed back by the UE.
It can be seen from the above description that, in the HARQ-ACK feedback mechanism defined by existing standards, the HARQ-ACK bit sequence finally fed back by the UE may include a HARQ-ACK bit corresponding to a downlink subframe which has no downlink HARQ transmission. For example, in the FDD system, whether or not there is downlink HARQ-ACK, transmission on the carrier, the HARQ-ACK bit sequence fed back by the UE always includes a HARQ-ACK bit corresponding to that carrier. In the TDD system, however, the UE determines the number of downlink subframes having downlink HARQ transmission on each carrier, but the value of Bc may be larger than the number of downlink subframes actually having downlink HARQ transmission in the time-frequency bundling window corresponding to the carrier. According to the current standards, the UE supports at most 5 carriers. Therefore, the existence of the nonsense HARQ-ACK bit does not have much impact to the system performance.
However, in order to further increase the downlink peak rate of the UE, it is well recognized by third generation partnership project (3GPP) member companies that the maximum number of carriers supported by the UE should be increased. According to the conclusion of the 3GPP radio access network (RAN) #66 meeting, the number of carriers supported by the UE will be increased to 32, wherein carriers on the unlicensed band may be included. With the increase of the number of downlink carriers supported by the UE, the absolute value of non-scheduled downlink subframes may increase accordingly. Therefore, if the current HARQ-ACK feedback mechanism is still utilized, the efficiency for feeding back information will decrease and finally affect the downlink peak rate of the UE, which contradicts the initial objective of increasing the number of carriers.
It can be seen from the above that, in order to effectively support downlink CA with at most 32 carriers, the existing HARQ-ACK feedback mechanism affects the downlink peak rate of the UE due to its low feedback efficiency. There is no appropriate solution at present.
The above information is presented background information only, to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.